In recent years, for example, communication traffic of wireless signals of mobile terminals is increased in accordance with an increase in mobile terminals, such as smartphones. Accordingly, by using optical networks, for example, large capacity optical transmission of wireless signals of mobile terminals performed among base stations is desired.
Furthermore, in a transmission technique for optical networks, for example, a multicarrier modulation technique, such as a discrete multi-tone (DMT) modulation technique or the like, is known. The DMT modulation technique is one of multicarrier transmission technologies based on the orthogonal frequency division multiplexing (OFDM) technology. The DMT modulation technique is a technology that allocates information signals to subcarriers (SCs) with different frequencies, performs multi-level modulation on the information signals allocated to the SCs, and transmits the information signals as DMT signals at a high speed.
Furthermore, for transmission devices in optical networks, there is a known method of multiplexing the frequencies of DMT signals into wireless signals of mobile terminals. A transmission device performs frequency multiplexing on wireless signals into DMT signals, performs electrical to optical conversion on the DMT signals obtained by performing the frequency multiplexing on the wireless signals, and transmits optical DMT signals to a transmission device on an opposite side via an optical network. At this time, the DMT signals use, for example, the frequency band of several GHz to about 25 GHz, whereas the wireless signals use, for example, the frequency band of about 700 MHz to about 6 GHz. The transmission device on the opposite side separates the information signals and the wireless signals from the optical DMT signals received from the optical network. Consequently, the transmission device on the opposite side can acquire the information signals and the wireless signals from the optical DMT signals.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-112781
FIG. 15A and FIG. 15B are a schematic diagram illustrating an example of the number of bits to be allocated for each SC (frequency) in a DMT signal. FIG. 15A is a schematic diagram illustrating an example of the number of bits to be allocated for each SC in a DMT signal in which a wireless signal is not multiplexed. FIG. 15B is a schematic diagram illustrating an example of the number of bits to be allocated for each SC in a DMT signal in which a wireless signal is multiplexed. Furthermore, FIG. 15A and FIG. 15B illustrate the result of the experiment obtained when transmitting, by using an optical transmission line with the length of 10 km, an optical DMT signal obtained by multiplexing the wireless signal of 2 GHz bandwidth into the DMT signal of 100 Giga Bit Ethernet (GbE (registered trademark)).
In the optical DMT signal indicated in FIG. 15A, from among a plurality of SCs in the DMT signal, the transmission characteristic of the SCs on the low frequency side is favorable and the transmission characteristic of the SCs on the high frequency side is gradually degraded. Furthermore, for the DMT signal indicated in FIG. 15A, because the wireless signal of the low frequency band is not subjected to frequency multiplexing, the transmission characteristic thereof is favorable. At this time, the transmission rate is 113 bit per second (Gbps) and the bit error rate (BER) of 6.57×10−4 is also obtained as the result of the experiment.
In contrast, in the optical DMT signal indicated in FIG. 15B, because the SCs on the low frequency side in which the transmission characteristic thereof is favorable overlap the wireless signal, the transmission characteristic thereof is remarkably decreased. At this time, the transmission rate is 109 Gbps and the BER of 8.32×10−4 is also obtained as the result of the experiment.
Namely, for a transmission device, when frequency multiplexing (multiplexing) is performed on a wireless signal into a DMT signal, because the carrier frequency of the wireless signal overlaps the SCs on the low frequency side in the DMT signal, in which the transmission characteristic is favorable, the transmission characteristic of the DMT signal is decreased.